About the only thing you can do is post a warning/disclaimer in the description to you material (that the mpeg 4 is bad, PS- check you're steaming files, they go slow often too).

First let me say I love this site. It's great and I appreciate all that they do here...

But...

This sound slow down happens a lot when they do the conversion to other codecs. Why not check the new generated files before posting? It seems like a huge waste of server space. Maybe I'm missing something, but come on, a little quality control goes a long ways.